You can go to the Calbre website and download the version that works on your Mac. I have to use 3.48 on my MacBook.
"If you are using macOS older than 10.14 (Mojave), the last version of calibre that will work on your machine is 3.48.
If you are using macOS 10.8 (Mountain Lion), the last version of calibre that will work on your machine is 2.85.1."
https://calibre-ebook.com/download_osx